Overvoltage-undervoltage sensor

ABSTRACT

A voltage sensitive device controls the energization of a relay or other load so as to switch it between energized and deenergized states as the sensed voltage passes a low or undervoltage limit and another high or overvoltage limit. Detection of the high and low limits is achieved by two voltage detecting circuits, one for each limit, and a comparator circuit responsive to the outputs of the detecting circuits performs the actual switching of the load between its energized and deenergized states. The two detecting circuits and the comparator circuit are each comprised principally of a programmable unijunction transistor or other three-terminal semiconductor device of generally similar characteristics.

United States Patent Tenenbaum 1 July 25, 1972 54]OVERVOLTAGE-UNDERVOLTAGE 3,354,448 1 1/1967 Brolin ..307/235 x SENSORPrimary Examiner.lohn Zazworsky [72] Inventor. glgcnea M. Tenenbaum, NewHaven, Aomey McCormick Paulding & Huber [73] Assignee: Allied ControlCompany, Inc., Plantsville, [57] ABSTRACT C A voltage sensitive devicecontrols the energization of a relay [22] Filed: June 9, 1971 or otherload so as to switch it between energized and de-ener- [21 APPL NO:151,190 gize d states as the sensed voltage passes a low or undervoltagelimit and another high or overvoltage limit. Detection of the high andlow limits is achieved by two voltage detecting cir- U-S- F uits one foreach [knit and a comparator ircuit esponsiv [51] Int. Cl. ..H03k 5/20 tothe outputs f the detecting circuits performs the m [58] Field of Search..307/235, 252 F Switching of the load between s energized and deencrgized states. The two detecting circuits and the comparator circuit[56] References Cited are each comprised principally of a programmableunijunction UNITED STATES PATENTS transistor or other three-terminalsemiconductor device of generally similar characteristics. 3,099,0007/1963 Dunning ..307/235 X 3,139,562 6/1964 Freeborn ..307/235 X 17Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION This inventionrelates to devices for controlling the energization of relays or otherelectrically energized controlled elements or loads in accordance withthe value of a particular sensed voltage, and deals more particularlywith such a device in the nature of an undervoltage-overvoltage sensorwhereby the controlled element is held in one state of energization whenthe sensed voltage is under a given low-limit voltage or over a givenhigh-limit voltage and is held in its other state of energization whenthe sensed voltage is of a value falling between such high and lowlimits.

In many instances it is desired to control the energization of a relayor the like in such a manner that it is held in one state ofenergization only when a given sensed voltage falls betweenpredetermined high and low limits. For example, in the case of a relay,it is often desired to have the relay energized when the sensed voltageis of a value falling between the high and low limits and de-energizedwhen the sensed voltage is below the low limit or above the high limit.Assuming this to be the case, a device accomplishing the desired controlfunction should detect the applied voltage and when the voltage is belowthe low limit should hold the relay in a de-energized state. As theapplied voltage rises to the low limit, the device should switch therelay to its energized state and the relay should be held in itsenergized state until the applied voltage rises to and reaches the highlimit at which time the device should operate to switch the relay to itsde-energized state and to hold the relay in such de-energized state asthe applied voltage rises above the high limit. As the applied voltagefalls from a point above the high limit a device performing the functionof this invention should operate in the reverse manner to switch therelay from its de-energized to its energized state as the appliedvoltage falls to the high limit and to then again switch the relay fromits energized to its de-energized state as the applied voltage reachesand passes the low limit.

Various devices for performing the above-described function have beenproposed and used in the past, however, most of those which have beenmade available use one or more relays as part of the control system,together with an associated electronic circuit, and are otherwiserelatively expensive and complicated.

The general purpose of this invention is to provide an improved andsimple overvoltage-undervoltage sensor which performs theabove-described function with the use of only a few inexpensivecomponents and which device is highly flexible with regard to adjustmentof the high and low limits of the voltage band to which it is sensitive.

SUMMARY OF THE INVENTION This invention resides in a voltage sensitivedevice, sometimes referred to as an undervoltage-overvoltage sensor, forswitching a relay or other controlled element between deenergized andenergized states as the voltage applied to the device passes, in eitherdirection, two separate high and low limit voltages. Low-level andhigh-level detecting circuits sense the applied voltages and produceoutput voltages indicative of whether the applied voltage is above orbelow the associated limit voltage, and a comparator circuit connectedto the outputs of the detecting circuits controls the actualenergization of the controlled element. The two detecting circuits eachare comprised of a programmable unijunction transistor, or similarthree-terminal semiconductor device, having its anode connected to theintermediate point of an associated voltage divider connected across theinput terminals of the device, and having its gate connected to areference voltage so that when the anode voltage rises to a levelbearing a given ratio to the gate voltage the semiconductor device isswitched from a non-conducting to a conducting state. The comparatorcircuit also preferably consists of a programmable unijunctiontransistor, or similar three-terminal semiconductor device, having itsanode and gate connected respectively to the outputs of the low and highlimit detectors so as to be held in a conducting state only when thesemiconductor devices of the two detecting circuits are both inconducting states. The relay or other controlled device may be connectedin series with the comparator circuit so as to be directly energized bythe anodecathode voltage of its semiconductor device, or it may becoupled to the comparator circuit through one or more stages ofamplification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing thefunction of a device embodying this invention.

FIG. 2 is a schematic wiring diagram illustrating a device embodyingthis invention.

FIG. 3 is a schematic diagram illustrating a device comprising anotherembodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION Turning now to the drawings, andfirst considering FIG. I, this figure shows the type of function whichthe device of this invention is intended to perform. It is assumed inthis figure that the controlled device is a relay which is switchedbetween off and on, or de-energized and energized, conditions as aparticular sensed voltage V varies in value. More particularly, V,represents a low limit of the sensed voltage and V represents a highlimit of such voltage. When the sensed voltage is less than the lowlimit V, the relay is held in its off condition. As the sensed voltagerises to and reaches the low limit V, the relay is switched rapidly toits on condition. As the sensed voltage rises beyond the low limit V,the relay remains in its on condition until the sensed voltage reachesthe high limit voltage V at which point the relay is rapidly switched toits off condition. Thereafter as the sensed voltage increases beyond thehigh limit the relay remains in its off condition. As the sensed voltagefalls from a value higher than the high limit voltage V the reverse formof switching takes place. That is, when the high limit V is reached therelay is switched from its off to its on condition and thereafter whenthe low limit voltage V, is reached the relay is switched from its on toits off condition. It will, of course, be understood that the device ofthis invention may be used to control the energization of othercontrolled elements or loads besides relays and that if desired, by theuse of inverters, the energization scheme of the controlled element maybe the inverse of that shown in FIG. 1.

Turning now to FIG. 2, this figure illustrates in schematic form adevice comprising one embodiment of this invention. In general, thedevice includes a low-level detecting circuit connected across its inputterminals l0, l0 and operable to switch from one condition to another asthe applied voltage passes the low limit, a high-level detecting circuitalso connected across its input terminals 10, 10 and operable to switchfrom one condition to another as the applied voltage passes the highlimit, and a comparator circuit connected with the outputs of the twodetecting circuits for energizing and deenergizing the controlledelement in response to changes in the combination of voltages appearingat the outputs of the two detecting circuits. The controlled element inthis case is shown to be a relay 12.

The low-level detecting circuit of the device of FIG. 2 includes athree-terminal semiconductor device Q, which is a pnpn structure andwhich is of the kind called both a programmable unijunction transistorand a complementary SCR. In cases where the device is designed andintended mainly for applications where it replaces and functionsgenerally similarly to a conventional unijunction transistor it is mostoften referred to as a programmable unijunction transistor. In theillustrated application the device 0,, as well as the devices Q and Qhereinafter mentioned, do function generally similar to unijunctiontransistors and are therefore referred to herein as programmableunijunction transistors or PUTs. For a more detailed understanding ofprogrammable unijunction transistors, reference may be had to GeneralElectric Application Note No. 9070-1 l/67 entitled The DI 3T-AProgrammable Unijunction Transistor. It should also be understood thatin its broader aspects the invention is not limited to the illustratedsemiconductor devices 0,, Q and Q being PUTs, but that if desired theycould also be conventional unijunction transistors or otherthree-terminal semiconductor device of the type which with regard to afirst two of its terminals has non-conducting and conducting states andwhich switches from the non-conducting state to the conducting statewhen the voltage applied across its said first two terminals risesrelative to the voltage applied to" its third terminal to a point, theintrinsic standoff point, at which the voltage across the first twoterminals equals the voltage applied to the third terminal times acertain proportionality factor, the intrinsic standoff ratio.

The anode of the PUT Q, is connected to an intermediate point 14 of avoltage divider connected across the input terminals 10, 10, the dividerconsisting of a fixed resistance 16 and a variable resistance 18. Areference voltage is applied to the gate of Q, by a zener diode 20connected between the negative terminal and the gate and a resistance 22connected between the gate and the positive terminal 10. The cathode ofQ, is connected to the negative terminal 10 through a droppingresistance 24, the side of the resistance opposite from the negativeterminal 10 constituting the output terminal of the low-level detectingcircuit. The zener diode and resistance 22 are so selected that thereference voltage applied to the gate of Q, is substantially less thanthe minimum low-limit voltage V, to which the device is to be sensitive,and by adjusting the adjustable resistor 18 the low-limit voltage V, maybe set to various different values. Therefore, when the applied voltageV rises to and reaches the low-limit voltage V, the voltage applied tothe anode of Q, reaches the intrinsic standoff point relative to thereference voltage applied to its gate and Q, is switched to itsconducting state.

vThe high-limit detector of the device of FIG. 2 is generally similar tothat of the low-limit detector and comprises a PUT Q having its anodeconnected to the intermediate point 26 of another voltage dividerconnected across the input terminals 10, 10 and consisting of a fixedresistance 28 and a variable resistance 30. The gate of Q is connectedto the cathode of the zener diode 20 so as to receive the same referencevoltage as 0,, and the cathode of O is connected to the negativeterminal 10 through a dropping resistance 32, the side of the resistance32 opposite from the negative terminal 10 constituting the outputterminal of the high-level detecting circuit. The high-limit voltage Vis established by adjusting the variable resistor 30 and when theapplied voltage V rises to and reaches the value V the voltage appliedto the anode of Q reaches the intrinsic standoff point relative to itsgate voltage and O is switched from its non-conducting to its conductingstate causing current to flow through the dropping resistor 32.

The comparator circuit of the device of FIG. 2 consists of a third PUT0,, having its anode connected to the output terminal of the low-leveldetector, the positive side of the resistance 24, and its gate connectedto the output terminal of I the high-level detector, the positive sideof the resistance 32.

The cathode of Q is connected to the negative input terminal 10 throughthe coil of the relay 12.

Considering the operation of the comparator circuit, when both Q, and Q,are non-conducting as a result of the applied voltage V being less thanthe low-limit voltage V, the anode, gate and cathode of Q are all heldat the negative voltage of the negative input terminal 10 and noconducting takes place through 0,, or the relay I2. When the PUT Q, isswitched to its conducting state, as a result of the applied voltagereaching the low-limit voltage V,, a positive voltage is applied to theanode of Q and, since the gate of Q, at this time has the voltage of thenegative terminal 10 applied to it, Q, is switched to a conducting stateand the relay [2 is energized. The relay 12 remains energized as theapplied voltage rises until the applied voltage reaches the high-limitvoltage V-,. At this time Q, is switched to its conducting state and apositive voltage is applied to the gate of Q The values of the droppingresistors 24 and 32 and other related components are so chosen that thisvoltage applied to the gate of 0;, is of a high level relative to thevoltage applied to its anode and sufficient to switch Q, to itsnon-conducting state, thereby de-energizing the relay [2.

As the applied voltage falls from a high value greater than thehigh-limit voltage V, the reverse procedure takes place. That is, as theapplied voltage reaches the high-limit voltage V the PUT Q of thehigh-level detecting circuit is switched to its non-conducting statethereby removing the gate voltage from Q, causing Q, to be switched toits conducting state and to energize the relay 12. When the appliedvoltage reaches the low-limit V,, the PUT Q, of the low-level detectorswitched to its non-conducting state which removes the positive voltagefrom the anode of Q and thereby returns Q, to its non-conducting stateto de-energize the relay 12.

In the device of FIG. 2 the relay I2 is directly energized by thecathode current of the comparator circuit PUT Q If the power required bythe controlled element is more than can be handled by the PUT (2,, thenthe controlled element may be coupled with the comparator circuitthrough one or more stages of amplification. FIG. 3, for example, showsa device embodying the invention wherein the controlled element, in theform of a relay 34, is coupled with the comparator circuit through onestage of amplification. In FIG. 3 all of the components which are or maybe identical to those of the device of FIG. 2 have been given the samereference numerals and need not be redescribed. The FIG. 3 device isidentical to that of FIG. 2 except that the relay 12 of FIG. 2 isreplaced by a dropping resistor 36 which supplies a signal to theamplifying circuit consisting of a transistor 0,, and resistor 38.

It will also be noted that in the device of FIG. 3 the load current forthe amplification stage is supplied from the same source, terminals10,10, as the high and low level detecting circuits. This is not,however, essential to the invention and, if desired, the amplificationstage or stages may be supplied with current from a different energysource, thereby increasing the capability of the device. The use of aseparate source for the amplification stage or stages is, for example,often desirable in cases where the signal source supplying the inputterminals 10,10 has a relatively high output impedance, that is does nothave a sufficient output energy capability to satisfactorily meet theload requirement.

I claim:

1. A voltage sensitive device for switching a controlled element betweende-energized and energized states as the voltage applied to said devicepasses two different voltage levels, said device comprising two inputterminals across which the sensed voltage is applied, a low-leveldetecting circuit connected across said two input terminals, saidlow-level detecting circuit having an. output terminal and beingoperable to cause the voltage appearing at said output terminal to be ofone value when said applied voltage is below a predetermined low leveland to be of a second value when said applied voltage is above saidpredetermined low level, a high-level detecting circuit connected acrosssaid two input terminals, said high-level detecting circuit having anoutput terminal and being operable to cause the voltage appearing atsaid latter output terminal to be of one value when said applied voltageis below a predetermined high level and to be of a second value whensaid applied voltage is above said predetermined high level, and acomparator circuit connected to both of said output terminals and tosaid controlled element for energizing and de-energizing said controlledelement in response to changes in the combination of voltages appearingat said two output terminals, each of said detecting circuits includinga three-terminal semiconductor device of the type which with regard to afirst two of its terminals has non-conducting and conducting states andwhich switches from said non-conducting state to said conducting statewhen the voltage applied across said first two terminals rises relativeto the voltageapplied to its third terminal to a point at which saidvoltage across said first two terminals equals said voltage applied tosaid third terminal times a given proportionality factor, means forapplying a proportion of said applied voltage to one of said first twoterminals of each of said two semiconductor devices, two droppingresistances, and means connecting the other one of said first twoterminals of each of said two semiconductor devices to one of said inputterminals through a respective one of said dropping resistances the sideof which resistance opposite from said one input terminal constitutessaid output terminal of the associated detecting circuit, and means forapplying a reference voltage to the third terminal of said semiconductordevice of each of said detecting circuits.

2. A voltage sensitive device as defined in claim 1 furthercharacterized by said comparator circuit including a third threeterminalsemiconductor device of the type which with regard to a first two of itsterminals has non-conducting and conducting states and which switchesfrom said non-conducting state to said conducting state when the voltageapplied across said first two terminals rises relative to the voltageapplied to its third tenninal to a point at which said voltage acrosssaid first two terminals equals said voltage applied to said thirdterminal times a given proportionality factor, said third semiconductordevice having its third terminal connected to the output terminal ofsaid high-level detecting circuit and means defining a circuit betweenthe output terminal of said low-level detecting circuit and said oneinput temtinal and passing through said first two terminals of saidthird semiconductor device which circuit is coupled with said controlledelement so as to cause said controlled element to assume one of its saidstates when current passes through said latter circuit as a result ofsaid third semiconductor being in a conducting state and to cause saidcontrolled element to assume the other of its said states when nocurrent flows through said latter circuit as a result of said thirdsemiconductor device being in its non-conducting state.

3. A voltage sensitive device as defined in claim 2 furthercharacterized by said latter circuit comprising means connecting one ofsaid first two terminals of said third semiconductor device to saidoutput terminal of said low-level detecting circuit and means connectingthe other one of said two terminals of said third semiconductor deviceto said one input terminal through said controlled element.

4. A voltage sensitive device as defined in claim 2 furthercharacterized by said latter circuit including means connecting one ofsaid first two terminals of said third semiconductor device to theoutput terminal of said low-level detecting circuit and means connectingthe other one of said first two terminals of said third semiconductordevice to said one input terminal through a dropping resistance, and anamplification stage responsive to the voltage appearing across saidlatter dropping resistance for controlling the energization of saidcontrolled element.

5. A voltage sensitive device as defined in claim 1 furthercharacterized by said semiconductor devices of said two detectingcircuits being programmable unijunction transistors.

6. A voltage sensitive device as defined in claim 2 furthercharacterized by said two semiconductor devices of said two detectingcircuits and said third semiconductor device of said comparator circuitall being programmable unijunction transistors.

7. A voltage sensitive device for switching a controlled element betweende-energized and energized states as the voltage applied to said devicepasses two different voltage levels, said device comprising two inputterminals across which the sensed voltage is applied, a low-leveldetecting circuit connected across said two input terminals, saidlow-level detecting circuit having an output terminal and being operableto cause the voltage appearing at said output terminal to be of onevalue when said applied voltage is below a predetermined low level andto be of a second value when said applied voltage is above saidpredetermined low level, a high-level detecting circuit connected acrosssaid two input terminals, said high-level detecting circuit having anoutput terminal and being operable to cause the voltage appearing atsaid latter output terminal to be of one value when said applied voltageis below a predetermined high level and to be of a second value whensaid applied voltage is above said predetermined high level, and acomparator circuit connected to both of said output terminals and tosaid controlled element for energizing and de-energizing said controlledelement in response to changes in the combination of voltages appearingat said two output terminals, each of said detecting circuits includinga voltage divider connected across said two input terminals and havingan intermediate tap point, each of said detecting circuits furtherincluding a three-terminal semiconductor device which with regard to afirst two of its terminals has non-conducting and conducting states andwhich switches from said non-conducting state to said conducting statewhen the voltage applied across said first two terminals rises relativeto the voltage applied to its third terminal to a point at which saidvoltage across said first two terminals equals said voltage applied tosaid third terminal times a given proportionality factor, and each ofsaid detecting circuits further including means connecting one of saidfirst two terminals of said semiconductor device to said tap point ofthe associated voltage divider and means connecting the other one ofsaid first two terminals of said semiconductor device to one of saidinput terminals through a dropping resistance the side of whichresistance opposite from said one input tem'tinal constitutes saidoutput terminal for said detecting circuit, and means for applying areference voltage to the third terminal of said semiconductor device ofeach of said detecting circuits.

8. A voltage sensitive device as defined in claim 7 furthercharacterized by said means for applying a reference voltage to one ofsaid two detecting circuits comprising a zener diode connected betweensaid third terminal of said semiconductor device of said one detectingcircuit and said one input terminal and a resistance connected betweensaid third terminal of said latter semiconductor device and the other ofsaid input terminals.

9. A voltage sensitive device as defined in claim 7 furthercharacterized by said two semiconductor devices of said two detectingcircuits having their third terminals connected to one another, and saidmeans for applying a reference voltage to the third terminal of thesemiconductor device of each of said detecting circuits comprising azener diode connected between said two third terminals and said oneinput terminal and a resistance connected between said two thirdterminals and the other one of said input terminals whereby the samereference voltage is applied to both of said semiconductor devices.

10. A voltage sensitive device for switching a controlled elementbetween de-energized and energized states as the voltage applied to saiddevice passes two different voltage levels, said device comprising twoinput terminals across which the sensed voltage is applied, twoprogrammable unijunction transistors, means connected with said twoinput terminals for applying a first proportion of said applied voltageto the anode of the first one of said two programmable unijunctiontransistors and for applying a second proportion of said applied voltageto the anode of the second one of said programmable unijunctiontransistors, said first programmable unijunction transistor having itscathode connected to the more negative one of said input terminalsthrough a first dropping resistance and said second one of saidprogrammable unijunction transistors having its cathode connected tosaid more negative one of said input terminals through a second droppingresistance, means for applying a reference voltage to the gate of eachof said two programmable unijunction transistors, and a comparatorcircuit connected to the more positive sides of said two droppingresistances and to said controlled element for energizing andde-energizing said controlled element in response to changes in thecombination of voltages appearing across said two dropping resistances.

11. A voltage sensitive device as defined in claim 10 furthercharacterized by said comparator circuit comprising a third programmableunijunction transistor having its anode connected to the positive sideof said first dropping resistance, having its gate connected to thepositive side of said second dropping resistance, and having its cathodeconnected to said more negative one of said input terminals through aload circuit coupled with said controlled element.

12. A voltage sensitive device as defined in claim 1 l'furthercharacterized bysaid load circuit including said controlled 14. Avoltage sensitive device as defined in claim 10 further characterized bysaid means connected with said input terminals for applying a firstproportion of said applied voltage to the anode terminal of the firstone of said programmable unijunction transistors and for applying asecond proportion of said applied voltage to the anode terminal of thesecond one of said programmable unijunction transistors comprising afirst voltage divider connected across said two input terminals andhaving an intermediate tap point connected to the anode of said firstprogrammable unijunction transistor and a second voltage dividerconnected across said two input terminals and having an intermediate tappoint connected to the anode of said second programmable unijunctiontransistor.

15. A voltage sensitive device as defined in claim 14 furthercharacterized by each of said two voltage dividers including oneadjustable resistance whereby the proportion of the applied voltagesupplied to the anode of the associated programmable unijunctiontransistor may be varied.

16. A voltage sensitive device as defined in claim 10 furthercharacterized by said two programmable unijunction transistors havingtheir gates connected to one another and said means for applying areference voltage to the gate of each of said two programmableunijunction transistors comprising a zener diode connected between saidtwo gates and the more negative one of said input terminals and aresistance connected between said two gates and the other of said inputterminals whereby the same reference voltage is applied to bot of saidgates.

17. A voltage sensitive device for switching a controlled elementbetween de-energized and energized states as the voltage applied to saiddevice passes two different voltage levels, said device comprising twoinput terminals across which the sensed voltage is applied, two voltagedividers connected across said two input terminals, two programmableunijunction transistors each having its anode terminal connected to anintermediate tap point of a respective one of said voltage dividers,said first programmable unijunction transistor having its cathodeconnected to the more negative one of said input terminals through afirst dropping resistance and said second one of said programmableunijunction transistors having its cathode connected to said morenegative one of said input terminals through a second droppingresistance, said two programmable unijunction transistors having theirgates connected directly to one another, a zener diode connected betweensaid two gates and said more negative one of said input terminals and aresistance connected between said two gates and the other one of saidinput terminals so as to apply a common reference voltage to said twogates, and a third programmable unijunction transistor having its anodeconnected to the positive side of said first dropping resistance and itsgate connected to the positive side of said second dropping resistanceand having its cathode connected to said more negative one of said inputterminals through a load circuit coupled with said controlled element.

1. A voltage sensitive device for switching a controlled element betweende-energized and energized states as the voltage applied to said devicepasses two different voltage levels, said device comprising two inputterminals across which the sensed voltage is applied, a low-leveldetecting circuit connected across said two input terminals, saidlow-level detecting circuit having an output terminal and being operableto cause the voltage appearing at said output terminal to be of onevalue when said applied voltage is below a predetermined low level andto be of a second value when said applied voltage is above saidpredetermined low level, a high-level detecting circuit connected acrosssaid two input terminals, said high-level detecting circuit having anoutput terminal and being operable to cause the voltage appearing atsaid latter output terminal to be of one value when said applied voltageis below a predetermined high level and to be of a second value whensaid applied voltage is above said predetermined high level, and acomparator circuit connected to both of said output terminals and tosaid controlled element for energizing and de-energizing said controlledelement in response to changes in the combination of voltages appearingat said two output terminals, each of said detecting circuits includinga three-terminal semiconductor device of the type which with regard to afirst two of its terminals has non-conducting and conducting states andwhich switches from said non-conducting state to said conducting statewhen the voltage applied across said first two terminals rises relativeto the voltage applied to its third terminal to a point at which saidvoltage across said first two terminals equals said voltage applied tosaid third terminal times a given proportionality factor, means forapplying a proportion of said applied voltage to one of said first twoterminals of each of said two semiconductor devices, two droppingresistances, and means connecting the other one of said first twoterminals of each of said two semiconductor devices to one of said inputterminals through a respective one of said dropping resistances the sideof which resistance opposite from said one input terminal constitutessaid output terminal of the associated detecting circuit, and means forapplying a reference voltage to the third terminal of said semiconductordevice of each of said detecting circuits.
 2. A voltage sensitive deviceas defined in claim 1 further characterized by said comparator circuitincluding a third three-terminal semiconductor device of the type whichwith regard to a first two of its terminals has non-conducting andconducting states and which switches from said non-conducting state tosaid conducting state when the voltage applied across said first twoterminals rises relative to the voltage applied to its third terminal toa point at which said voltage across said first two terminals equalssaid voltage applied to said third terminAl times a givenproportionality factor, said third semiconductor device having its thirdterminal connected to the output terminal of said high-level detectingcircuit and means defining a circuit between the output terminal of saidlow-level detecting circuit and said one input terminal and passingthrough said first two terminals of said third semiconductor devicewhich circuit is coupled with said controlled element so as to causesaid controlled element to assume one of its said states when currentpasses through said latter circuit as a result of said thirdsemiconductor being in a conducting state and to cause said controlledelement to assume the other of its said states when no current flowsthrough said latter circuit as a result of said third semiconductordevice being in its non-conducting state.
 3. A voltage sensitive deviceas defined in claim 2 further characterized by said latter circuitcomprising means connecting one of said first two terminals of saidthird semiconductor device to said output terminal of said low-leveldetecting circuit and means connecting the other one of said twoterminals of said third semiconductor device to said one input terminalthrough said controlled element.
 4. A voltage sensitive device asdefined in claim 2 further characterized by said latter circuitincluding means connecting one of said first two terminals of said thirdsemiconductor device to the output terminal of said low-level detectingcircuit and means connecting the other one of said first two terminalsof said third semiconductor device to said one input terminal through adropping resistance, and an amplification stage responsive to thevoltage appearing across said latter dropping resistance for controllingthe energization of said controlled element.
 5. A voltage sensitivedevice as defined in claim 1 further characterized by said semiconductordevices of said two detecting circuits being programmable unijunctiontransistors.
 6. A voltage sensitive device as defined in claim 2 furthercharacterized by said two semiconductor devices of said two detectingcircuits and said third semiconductor device of said comparator circuitall being programmable unijunction transistors.
 7. A voltage sensitivedevice for switching a controlled element between de-energized andenergized states as the voltage applied to said device passes twodifferent voltage levels, said device comprising two input terminalsacross which the sensed voltage is applied, a low-level detectingcircuit connected across said two input terminals, said low-leveldetecting circuit having an output terminal and being operable to causethe voltage appearing at said output terminal to be of one value whensaid applied voltage is below a predetermined low level and to be of asecond value when said applied voltage is above said predetermined lowlevel, a high-level detecting circuit connected across said two inputterminals, said high-level detecting circuit having an output terminaland being operable to cause the voltage appearing at said latter outputterminal to be of one value when said applied voltage is below apredetermined high level and to be of a second value when said appliedvoltage is above said predetermined high level, and a comparator circuitconnected to both of said output terminals and to said controlledelement for energizing and de-energizing said controlled element inresponse to changes in the combination of voltages appearing at said twooutput terminals, each of said detecting circuits including a voltagedivider connected across said two input terminals and having anintermediate tap point, each of said detecting circuits furtherincluding a three-terminal semiconductor device which with regard to afirst two of its terminals has non-conducting and conducting states andwhich switches from said non-conducting state to said conducting statewhen the voltage applied across said first two terminals rises relativeto the voltage applied to its third terminal to a point at which saidvoltage across said first two terminals equals said voltage applied tosaid third terminal times a given proportionality factor, and each ofsaid detecting circuits further including means connecting one of saidfirst two terminals of said semiconductor device to said tap point ofthe associated voltage divider and means connecting the other one ofsaid first two terminals of said semiconductor device to one of saidinput terminals through a dropping resistance the side of whichresistance opposite from said one input terminal constitutes said outputterminal for said detecting circuit, and means for applying a referencevoltage to the third terminal of said semiconductor device of each ofsaid detecting circuits.
 8. A voltage sensitive device as defined inclaim 7 further characterized by said means for applying a referencevoltage to one of said two detecting circuits comprising a zener diodeconnected between said third terminal of said semiconductor device ofsaid one detecting circuit and said one input terminal and a resistanceconnected between said third terminal of said latter semiconductordevice and the other of said input terminals.
 9. A voltage sensitivedevice as defined in claim 7 further characterized by said twosemiconductor devices of said two detecting circuits having their thirdterminals connected to one another, and said means for applying areference voltage to the third terminal of the semiconductor device ofeach of said detecting circuits comprising a zener diode connectedbetween said two third terminals and said one input terminal and aresistance connected between said two third terminals and the other oneof said input terminals whereby the same reference voltage is applied toboth of said semiconductor devices.
 10. A voltage sensitive device forswitching a controlled element between de-energized and energized statesas the voltage applied to said device passes two different voltagelevels, said device comprising two input terminals across which thesensed voltage is applied, two programmable unijunction transistors,means connected with said two input terminals for applying a firstproportion of said applied voltage to the anode of the first one of saidtwo programmable unijunction transistors and for applying a secondproportion of said applied voltage to the anode of the second one ofsaid programmable unijunction transistors, said first programmableunijunction transistor having its cathode connected to the more negativeone of said input terminals through a first dropping resistance and saidsecond one of said programmable unijunction transistors having itscathode connected to said more negative one of said input terminalsthrough a second dropping resistance, means for applying a referencevoltage to the gate of each of said two programmable unijunctiontransistors, and a comparator circuit connected to the more positivesides of said two dropping resistances and to said controlled elementfor energizing and de-energizing said controlled element in response tochanges in the combination of voltages appearing across said twodropping resistances.
 11. A voltage sensitive device as defined in claim10 further characterized by said comparator circuit comprising a thirdprogrammable unijunction transistor having its anode connected to thepositive side of said first dropping resistance, having its gateconnected to the positive side of said second dropping resistance, andhaving its cathode connected to said more negative one of said inputterminals through a load circuit coupled with said controlled element.12. A voltage sensitive device as defined in claim 11 furthercharacterized by said load circuit including said controlled elementconnected in series with said cathode of said third programmableunijunction transistor and said more negative one of said inputterminals.
 13. A voltage sensitive device as defined in claim 11 furthercharacterized by said load circuit including a third dropping resistanceconnEcted in series with said cathode of said third programmableunijunction transistor and said more negative one of said inputterminals, and an amplification stage connected across said thirddropping resistance for controlling the energization of said controlledelement in response to the voltage appearing across said third droppingresistance.
 14. A voltage sensitive device as defined in claim 10further characterized by said means connected with said input terminalsfor applying a first proportion of said applied voltage to the anodeterminal of the first one of said programmable unijunction transistorsand for applying a second proportion of said applied voltage to theanode terminal of the second one of said programmable unijunctiontransistors comprising a first voltage divider connected across said twoinput terminals and having an intermediate tap point connected to theanode of said first programmable unijunction transistor and a secondvoltage divider connected across said two input terminals and having anintermediate tap point connected to the anode of said secondprogrammable unijunction transistor.
 15. A voltage sensitive device asdefined in claim 14 further characterized by each of said two voltagedividers including one adjustable resistance whereby the proportion ofthe applied voltage supplied to the anode of the associated programmableunijunction transistor may be varied.
 16. A voltage sensitive device asdefined in claim 10 further characterized by said two programmableunijunction transistors having their gates connected to one another andsaid means for applying a reference voltage to the gate of each of saidtwo programmable unijunction transistors comprising a zener diodeconnected between said two gates and the more negative one of said inputterminals and a resistance connected between said two gates and theother of said input terminals whereby the same reference voltage isapplied to both of said gates.
 17. A voltage sensitive device forswitching a controlled element between de-energized and energized statesas the voltage applied to said device passes two different voltagelevels, said device comprising two input terminals across which thesensed voltage is applied, two voltage dividers connected across saidtwo input terminals, two programmable unijunction transistors eachhaving its anode terminal connected to an intermediate tap point of arespective one of said voltage dividers, said first programmableunijunction transistor having its cathode connected to the more negativeone of said input terminals through a first dropping resistance and saidsecond one of said programmable unijunction transistors having itscathode connected to said more negative one of said input terminalsthrough a second dropping resistance, said two programmable unijunctiontransistors having their gates connected directly to one another, azener diode connected between said two gates and said more negative oneof said input terminals and a resistance connected between said twogates and the other one of said input terminals so as to apply a commonreference voltage to said two gates, and a third programmableunijunction transistor having its anode connected to the positive sideof said first dropping resistance and its gate connected to the positiveside of said second dropping resistance and having its cathode connectedto said more negative one of said input terminals through a load circuitcoupled with said controlled element.